This invention relates to an optical disc apparatus and, more particularly, to an optical disc apparatus used with an optical disc that has a spiral track or concentric tracks for recording and reproducing data.
The optical disc apparatus can record and reproduce a great quantity of data and is suited for recording and reproducing image data. Such optical disc apparatus is used with an optical disc that has a spiral track or concentric tracks. For the retrieval of a desired track an optical head is moved by a linear motor in a radial direction on the optical disc. Data is recorded on or reproduced from the retrieved track. During track retrieval, the optical head is positioned with respect to the optical disc by a position detector using an optical linear scale and an optical mask. With the optical disc apparatus of the type described, however, a mechanical error is sometimes introduced when installing the optical scale, and in such a case it is no longer possible to obtain accurate positioning of the optical head. To alleviate this drawback, one proposed method is to detect a scale value corresponding to the distance from an initial position of the scale to the innermost track of the optical disc and store the detected scale value as a corrected scale value in a memory. At the time a track is retrieved, i.e., when access is made thereto, the optical head is moved linearly until the scale value detected by the position detector coincides with the sum of a scale value corresponding to the intended track and the corrected scale value. The optical head is positioned in this manner.
However, if a scar or scratch is formed on a portion of the optical disc between the center and the innermost track or if there is a dust particle attached to that position of the disc, such a scar or scratch or dust particle is misjudged as the innermost track. In such a case, the accurate corrected scale value, and hence accurate positioning of the optical head, cannot be obtained.